1. Field of the Invention
The present invention relates to an electric signal transfer element which utilises ultrasonic waves, and more particularly, to an electric signal transfer element wherein spuriouses of odd multiplication reflected waves of transfer electric signals are attenuated.
2. Related Art Statement
Where electric insulation between circuits or between appliances is effected while transferring electric signals, it has heretofore been well known that a photo coupler is employed as an isolator.
This photo coupler which is molded into a single package is, as illustrated in FIG. 5, composed by a light emission element LED (Light Emission Diode) and a light receiving element CdS (or a photo transistor). The thus constructed photo coupler performs the insulation both on the input-side and on the output-side, in which case the light serves as a medium.
For this reason, there are yielded advantages wherein it is feasible to freely arrange a power source voltage (including a bias) between the appliances and to select an earthing point for the two systems. Moreover, noises can be reduced.
This type of photo coupler is, however, attended with the following disadvantages. This photo coupler is slow of frequency responding velocity, and a transferable pulse repeating frequency of an inexpensive general purpose photo coupler is for now 1 MHz or less. Other types of photo couplers the frequency of which is 10 mHz or thereabouts are put on the market, but they are so expensive that they are not yet utilised in broadly used appliances.
The photo coupler transfers the signals in only one direction. For instance, when this photo coupler is employed for an interface of a microcomputer, totally two pieces of photo couplers are needed for transmitting/receiving signal wires, this undesirably conducing to an increase in cost.